Magnesium to be used as a raw material of a magnesium ion battery is an element present abundantly on the earth, and is a material having higher superiority as compared with lithium which is unstable in price and supply amount, or the like. In addition, since the magnesium ion battery is cheap and safe, as well as has high energy density, it has drawn attention as a post-lithium ion battery.
As a negative electrode of the magnesium ion battery, usually metal magnesium is used. However, since metal magnesium has high reducibility, in the case where said metal is used as a negative electrode, it reacts with an electrolytic solution to form a passive state film having low ion conductivity, at the electrode surface thereof. And, there has been known that formation of this passive state film inhibits reversible dissolution and deposition of magnesium, which has been a problem on using metal magnesium as a negative electrode.
On the other hand, an electrolytic solution not forming the passive state film has also been known. For example, in PATENT LITERATURE 1 and NON PATENT LITERATURE 1, there has been reported that by using an electrolytic solution, where an electrolyte represented by the general formula Mg(ZR11R2mXn)2 (wherein Z represents boron or aluminum; R1 and R2 represent a hydrocarbon group; X represents bromine or chlorine; and l+m+n is 4) is dissolved in tetrahydrofuran (THF), reversible dissolution and deposition of magnesium is possible.
Additionally, various reports have been made aiming at enhancing performance of the magnesium ion battery. For example, PATENT LITERATURE 2, there has been reported that low oxidation potential conventionally said for the Grignard's reagent (RMgX, wherein R is an alkyl group) can be improved by using an electrolytic solution of an aromatic Grignard's reagent represented by the general formula C6H5MgX (wherein X=Cl, Br) dissolved in tetrahydrofuran (THF).
In addition, in PATENT LITERATURE 3 and PATENT LITERATURE 4, there has been reported that by using the Grignard's reagent (RMGX) or magnesium (II) chloride in combination with an organometal compound (an alkylaluminum compound), a complex is formed by making magnesium dimerized in the system, and oxidation resistance of an electrolytic solution can be improved.
Still more, in NON PATENT LITERATURE 2, there has been a description on the progress of reversible dissolution and deposition of magnesium, from the result of cyclic voltammogram and electrode surface analysis, by preparation of an electrolytic solution wherein a magnesium (II) bromide is dissolved in 2-methyltetrahydrofuran in order to enhance safety of the electrolytic solution.